Communication by frequency variation



Aug, 18, 1931.

AHXXXXX F WER AMPL/F/ER F/Uik c. w. HANSELL 1,819,508

COMMUNICATION BY FREQUENCY VARIATION Him Filed Aug. 11. 1927 2 Sheets-Sheet 1 N N Q N N N w a P Q fl INVENTOR CLARENCE W. HANsELL BY AT ORNEY 11, 1927 2 Sheets-Sheet 2 C. W. HANSELL Filed Aug.

COMMUNICATION BY FREQUENCY VARIATION Patented Aug. 18, 1 931 UNITED s'rA'r|-:s

PATENT oFFIcE CL ARENCE IT. HANSELL, OF ROCKY POINT, NEW YORK, ASSIGNOR TO RADIO CORPORA- T1011 01 AMERICA, A CORPORATION OF DELAWARE coxmoNIcA'r'IoN' BY FREQUENCY VARIATION Application filed August 11, 1927. Serial .No. 212,192.

This invention relates to frequency moduling channels a difliculty encounteredis that lation, and to a method and means for multiplexing signals transmitted by frequency modulation.

In order toreduce fading it has been suggested thatthe frequency of transmission be wobbled. It is an object of my invention to transmit keyed signals by causing) an intermittent frequency wobble prefera ly during the marking periods of keying, and to detect the wobble at the receiver, thereby solving the problems of keying and of wobbl ng at the same time. This I do by keylng relatively low frequency energy and uslng the keyed energy to fre uency modulate transmission ener that 1s, the transmission energy has its requency varied at a frequency equal to the low frequency, and this frequency variation is unaccompanied by variations 1n g0 amplitude of the transmission energy. At the receiver an analyzing circuit tuned to a fre uency lying outside of the applled range of requency is used to change the frequency variations to amplitude variations.

:5 It is a further object of my invention to permitof multiplex communication by means of frequency variation, and this I do by se arately signalling on each of a plurality 0 signal energies of different relatively so low frequencies, adding the separate signal energies to obtain a complex resultant signal wave, and using the resultant wave to frequency modulate transmission energy of relatively high frequency. 7

To receive this multiplex frequency modulated carrier my method includes impressing the received energy upon an analyz ng circuit which is tuned to a frequency lylng outside of the operating range of frequency supplied to it in order to change the frequency modulation to amplitude modulation, rectif ing the amplitude modulated energy in or er to obtain a complex signal wave such as was used "for frequency modulation at the transmitter, separating the rectified energy into its component signal energies by means of filters, and then separatelytranslating the component signal energies to obtain the several original signals. L In selecting the frequencies for the signalquency as constant as possible.

these diiiiculties is a further object of my these frequencies must not be harmonically related. But if natural harmonics are supbe used in the signal channels. It is a property of balanced circuits that by their use alternate harmonic frequencies are supo pressed, energy of harmonic frequency may pressed, and a further object of my invention is to provide a balanced'frequency modulator, thereby' permitting a much wider choice of frequenciesfor the various signal energies. This device may be termed a push-' pull Wobbler, and in its broadest form com- 1 prises means for varying the tuning of a resonant circuit in one direction, means for varying the tuning of said resonant circuit in the opposite direction, and a modulator circuit arranged to energize said means in phase opposition, preferably through the use of electron emission tubes the control electrodes of which are coupled in push-pull.

One of the advantages of usin frequenc modulation in preference to amp itudemo ulatioti is the fact that the necessary frequency band for each transmitter may be reduced to any desired minimum. In usin a small frequency variation in a high requency carrier the percentage variation may be so small as to present modulating and demodulating difficulties, both at the transmitter and receiver. Furthermore, with small frequency Variations for the signal it becomes exceedingly important to keep the mean fre- To overcome invention, which I accomplish by using the signal energy of low frequency to frequency modulate energy of an intermediate frequency, thereby increasin the percentage variation, and then utilizing the intermediate frequency energy for modulating the high-frequency carrier of a single side band transmitter. In somewhat similar fashion the received wave is heterodyned to an intermediate frequency, in which the relative frequency variation may be made as at as desired, depending upon the magnitude of intermedate frequency chosen. With a transmitter arranged as Indicated the source of carrier frequency need not be Under unfavorable atmospheric conditions it is desirable to narrow'the' width of the frequency band employed for communication.

11 such case, in order to obtain the desired 7 amplitude of amplitude modulation at the receiver it becomes necessary to decrease the damping coefiicient of the analyzing circuit of the receiver. When this is done it may be found that the signal amplitude has not increased, the reason being that the more efficient resonant circuit tends to average out the frequencies applied to it. To overcome this we must either increase the wobble range, or decrease the wobble frequency. The former, obviously, would defeat the object" sought, which was to narrow the frequency range, so we must resort to the latter. Inasmuch as maximum keying speed depends upon the wobble frequency of the channel being keyed, it follows that it may further be necessary to slow up the keyin speed, but this may itself be not undesira le under unfavorable atmospheric conditions.

Accordingly, my invention includes adjusting the frequency band according to atmosheric conditions. It further includes adusting the damping coeflicient of the analyzing circuit of the receiver according to the width of frequency band, It still further includes adjusting the wobble frequency to avoid loss of signal amplitude by reason of the low decrement indicated by too sharp a resonance curve. On the other hand, in the case of favorable communication conditions my invention contemplates increasing the keying speed, and in the order of their necessity, increasing the wobble frequency at the transmitter, the damping 'coefiicient of the analyzing circuit at the receiver, and the wobble range at the transmitter.

Sometimes fading is experienced which only partially reduces the signal intensity, and which therefore might be thought not serious, but such fading may itself-occur at an audible frequency and for that reason prove ruinous to reception. This type of fading may be contended with when frequency modulated signals are transmitted by taking advantage of the difference in the character of the modulations produced in order to discriminate between them, the disturbance being an amplitude variation, whereas the signal is a frequency variation. V To eliminate the former and not the latter the amplifying tubes used in the receiver may be arranged as ated from a suitable antenna System24. 7

alimiting device, preferably'by over excitation of the grids of the vacuum tubes in one or more stages of the amplifier. Subsequent to the elimination of the amp 'tude varia tions the frequenc modulation ay be transposed to amplitu e variation inorder to obv tain the transmitted signals.

Further objects appear in the following more detailed specification, which is accompanied by drawings in which,

Figure 1 is a multi lex transmitter;

Figures 2 and 3 in icate alternative means for obtaining frequency variation;

Figure 4 shows a push-pull frequency modulator;

Figure 5 is a multiplex receiver; and

Figures 6, 7 and 8 are explanatory of the adjustments made in the receiver.

Referring to Figure 1 there are a plurality of sources of relatively low frequenc 86 energy, 2, of frequencies f f and f which in the case of telephony signals should be super audible frequencies. These signalling energies are separately controlled by signalling means, exemplified in this case by, the keys 4, but which may equally well be controlled by picture transmission relays, or microphones. The various channels are led to a common bus 6 from which the complex signalling energy is led to a suitable il means for obtaining frequency modulation. This has been indicated in the present-case by a magnetic Wobbler 8, the sole requirement for which is that it should be operated on a linear portion of its characteristic curve, I in order that the signals in the various channels may not be intermodulated.

The frequency modulation may take place directly in the transmission energy, or preferably it may take place first in intermediate). frequency energy which then may be usedto frequency modulate higher frequency energy for transmission. The latter scheme is here shown, and the intermediate frequency oscillator comprises an electron emission tube I" 12, which is regeneratively coupled through suitable blocking condensers 13 to a resoband transmitter 16. The carrier frequency! F may be crystal controlled by, a crystal 18,

and the side bands are separated by a filter 20, the output from which may be amplified in a power amplifier 22 before being radi- 1. The frequency modulation may be accomplished by capacity variation, instead of inductance variation, as is indicated in Figure 2, which represents a fragment of Figure 1 in which the magnetic Wobbler 8 has-ll It is more difficult to obtain linear frequency.

variation with this type of modulation, but

' themagnetic response of the armature may be so adjusted as to obtain the desired result. The variable condenser may be arranged in parallel with a fixed condenser, and their magnitudes relatively adjusted to control the extent of frequency modulation obtained.

Frequency modulation may also be obtained by variably detuning a resonant circuit through the use of a resistance shunt. This has been illustrated in" Figure 3 in which a portion of the inductance of the resonant circuit 14 is shown by-passed by an electron emission tube 32, the impedance of which is varied by fluctuations of its grid potential obtained by coupling the signalling energy bus 6, to the cathode and grid of the tube. The cathode to anode circuit of the tube 32 may be shunted, if desired, by a resistance 34, adjustment of which controls the extent of frequency variation obtained for given potential variations applied to the tube grid. In Figure 4 I have indicated a balanced frequency modulator. This comprises an inductive circuit 15 and a capacitive circuit 17 each controlled by an electron emissidn tube, arranged in parallel with one another and with a portion of the reactance of the resonant circuit 14. The condenser of circuit 17 is'by-passed by a radio frequency choke to permit direct anode potential to reach the tube anode. The modulating energy from the bus 6 is coupled to thecontrol electrodes of the tubes in phase opposition, and the tubes are made conductive only alternately. When the inductive circuit 15 is made effective as a shunt the reactance of the shunted portion.

of the tuning coil of the resonant circuit 14 is lowered,.whereas during the other half cycles when the capacitive circuit 17 is connected in parallel the reactance is raised. \Vith this balanced modulator the resonant frequency is made to fluctuate to both sides of the mean frequency of an efficient resonant circuit, which is preferable to detuning all in one direction. It is possible to obtain a more nearly linear characteristic, for only half as long a portion'of each tube characteristic need be utilized, and identical portions of the characteristics of the two tubes may be chosen. A still more important advantage is the suppression of undesired harmonics of the modulating frequencies.

Figure 5 is a wiring diagram for a receiver arranged to cooperate with the transmitter shown in Figure 1. The transmitted energy of mean frequency F +F is collected on a suitable antenna circuit 36 and applied to the input circuit 38 of a combining tube 40. local oscillator 42 is arranged to supply energy for heterodyning the receiver energy to any desired intermediate frequency. In the present case the oscillator'frequency is indicated as F in which case the intermediate frequency is F and is therefore equal to the transmitter intermediate frequency, but this is not an essential condition. Heterodyne reception is useful to beat down the carrier frequency without reducing the extent of frequency ariation, thereby making demodulation less difficult. It helps avoid fly-wheel effect, which is discussed later.

The energy of intermediate frequency is amplified in cascade amplifiers 44, before the stages of which there are interposed band pass filters 46, the purpose of which will be more fully explained in connection with Figure 6. The amplified intermediate frequency energy-is supplied to an analyzing circuit,

which in this case is the resonant input circuit 48 of the rectifying device 50. This circuit includes equal resistances 49, and a common contactor 51 for coupling the analyzing circuit symmetrically to the input circuit of the detector tube. damping resistance is to control the sharpness of the resonance curve of the circuit 48. The analyzing circuit 48 is tuned to a frequency which lies outside of the operating range of intermediate frequency, and therefore serves to change the frequency modulation to amplitude modulation. The amplitude modulated energy is rectified in a detector tube 50, the rectified output from The function of this which contains a complex wave composed 51.

of the low frequencies f f etc. These may be separated by the filters 52, the outputs from which are led to separate translating devices 54, herein shown as tele hone headsets, but whichmay obviously corders or other appropriate translating devices.

In Figures 6 the curve 56 represents the resonance curve of the analyzing circuit 48,

and it will be seen that its resonance fie- L.

e tape re-' I quency lies so far outside of the mean fre-' quency F that the operating range of frequency variation lies entirely on one sloping side of the resonance curve. The band pass filters 46 are adjusted to a range about as shown, and their chieffunction is to preclude frequencies lying on the other side of the resonance curve, altho they incidentally will preclude certain undesired frequencies A. lines 58. In order to obtain the same variation in current amplitude as is indicated by the vertical spacing between the points of intersection of the lines 58 with the resonance curve 56 it then is necessary to steepen the side of the resonance curve, b obtaining a new resonance curve, indicate by the curve 60. With a high wobble frequency the ex- Egctcd increase in current variation will ndt.

realized because of the averaging out or fly-wheel effect of the more efiicient resonant circuit, and therefore, as before explained, it may be necessary to decrease the wobble frequency at the transmitter. On the other hand, under conditions more favorable than the average, the keying speed may be so far increased as to necessitate an increase of wobble frequency, and then in order to avoid fly-wheel efiect the resistance 49 in Figure may be increased, causinga less sharp resonance curve,such as the curve 62 in Figure 6.

To again obtain full current variation the extent of frequency variation or wobble range may be increased by the operator at the transmitter, who in practice may chan e wobble range and wobble frequency simultaneously in the same direction.

In Figure 7' the curve70 indicates how a received signal ..may fade to a small extent but with sufiicient rapidity to itself constitute a disturbing wave. This type of fading may be overcome by introducing limiting means in the intermediate amplifier tubesso that saturation will occur at a signal level slightly below the lowest fading intensity. In Figure 8 a characteristic curve for a vacuum tube is indicated, the flattening of the top of which shows the typeof limiting which is ini prefer to overexcite the tubes, but this is troduced for a given anode potential.

merely for economy of apparatus, and I may instead use a reduced anode potential, or a limited filament emission. Obviously, in case fading reduces the signal to zero the expedient here suggested is futile.

It should be understood that altho I consider the use of a heterodyne to beat the transmission" frequency down to an intermediate 7 frequency exceedingly desirable, yet it is not tage, in the case of code signals, that there are no key clicks to interfere with broadcast receivers, in addition to the advantages of reduced fading and reduced requisite frequency range. The power amplifier of the transmitter runs at constant 5 load, thereby increasing the life of the vacuum tubes. Furtherinore the signalling is relatively secret, for the range of frequency variation may be made so small as to be incapable of reproducsignals in an ordinary receiver, and is especially so in the case of multiplex signals because the apparent signal, even if made audible, is unintelligible unless properly separated into its component signals.

I claim:

1. The method 'of transmitting intelligence which includes keying relatively low frequency energy and using the low frequency energy to vary the frequency of high fre-.

quency transmission energy at a fre uency equal to the low frequency, and with su stantially no variation in amplitude of the trans mission energy thereby eliminating keying clicks in apparatus within the range of trans mission.

2. The method of transmitting multiplex signals by frequency variation which in cludes signalling on each of a plurality of dif ferent relatively low frequency energies,

adding the signal energies, and using the resultan. complex wave to frequency modulate transmission energy according to a linear characteristic.

'3. The method of analyzing a multiplex frequency modulated constant amplitude carrier wave which includes impressing the vreceivedwave "on a resonant circuit which is tuned to a'frequency which lies outside of the operating frequenc fying the different requency portions of the impressed wave unequally, rectifying the resulting wave, separating the rectified wave into its. constituent signal channels, and

translating the energy of each channel.

fl. The method of analyzing a constant amplitude carrier wave which has been frequency modulated by a plurality of keyed range, thereby amplisignal energies which includes heterodyning the received wave to obtain frequency modulated energy of intermediate frequency, impressing the received wave on a resonant circuit which is tuned to a frequency which lies outside of the operating range of intermediate frequency, thereby amplifying the different frequency portions of the impressed wave unequally, rectifying the resulting wave, separating the rectified wave into its constituent signal energies, and separately translating the signal energies.

5. The method of transmitting intelligence which includes signalling on relatively low frequency energy, frequency modulating intermediate frequency energy with the signal energy, modulating high frequency energy with the intermediate frequency energy, and transmitting a side band of the modulated high frequency energy.

6. The method of transmitting multiplex signals by frequency variation which ineludes signalling on each of a plurality of signal energies of different relatively low frequencies, adding the signal ener 'es, frequency modulating intermediate requenc ener ywith the resultant signal energy, mo ulatmg high frequency energy with the in- -termediate frequency energy,and transmitting a side band of the modulated high freeach of a plurality of signal energies o includes utilizing alternate half cycles of the modulating energy to; connect inductive reactance in parallel with a portion of the resonant circuit, and the other half cycles of the modulating energy to connect capacitive reactance in parallel with a portion of the resonant circuit.

9. In communication by frequency modulation the method which includes decreasing the extent of frequency variation during unfavorable atmospheric conditions, and increasing the extent of frequency variation during favorable atmospheric conditions.

10. In communication by frequency modulation the method which includes widening or narrowing the frequency band to obtain most favorable communication, and increasing or decreasing the damping coefiicient of a circuit of the receiver according as the frequency band is wider or narrower.

11. In communication by frequency wobble, the method of improving communication during unfavorable atmospheric conditions which includes, at the transmitter decreasing the range and frequency of wobble, and at the receiver decreasing the damping coefiicient of an analyzing circuit.

12. The method of eliminating the effect ofpartial fading of frequency modulated signals which includes discriminating between amplitude and frequency variation by limiting the received signal tothe low signal level, and thereafter changing frequency modulation to amplitude modulation for detection.

13. A transmission system for. eliminating.

keying clicks and reducing fading comprising anoscillator having controlling means to determinethe oscillation frequency of, the oscillator, a source ofrelatively low frequency energy, means to vary the frequency controlling means in response to the low fre-- uency energy, and means requency ener 14. A multip ex transm ssion system comto key the low prising a plurality of sources of energy of different low frequencies, signalling means to control the energy of each frequency,

means to add the signal energies, a source of high frequency energy, and means to freguency modulate the high frequency energy in response to the resultant signal ener in accordance with a linear characteristic.

15. transmission system comprisin a single slde band transmitter, a source 0 intermediate frequency for modulatin the transmitter carrier energy, a source 0 relatively low frequency energy, means for fre quency modulating the intermediate frequency energy 1n response thereto, and signalllng means for controlling the low frequency energ A multiplex transmission system comprising a single side band transmitter, a source of intermediate frequency for modulating the transmitter carrier energy, a plurality of sources of energy of difierent relatively low frequencies, means-toadd the low frequency energies to obtain their resultant, means for frequency modulating the intermediate frequency energy in response to the resultant, and signalling means for separately signalling on each of the low frequency energies. v r

- l7. A'multi-plex'receiving system for constant amplitude frequency modulated energy including an analyzing circuit for chan 'ng frequency modulation to am litude mo ulation comprising a resonant circuit tuned to a frequency lying outside of the operating range of frequency supplied to it, a rectifying circuit, and means to separate the rectified wave into its component energies of different frequency.

18. A balanced frequency Wobbler comprising a resonant circuit tuned to the mean frequency, means responsive'to modulating energy for varying the tuning of the resonant circuit in one direction, means responsive to modulating energy for varying the tuning of the resonant circuit in the opposite direction, and a modulating circuit coupled to said means in phase opposition.

19. .A balanced frequency modulator including aresonant circuit tuned to the mean frequency, a plurality of electron emission tubes having control electrodes, means controlled by one tube for varying the tuning of the resonant circuit in onedirection, means controlled by another tube for varying the tuning of the resonant circuit in the opposite direction, and a modulating circuit coupled to the control electrodes of said tubes in phase opposition.

20. A balanced frequency modulator eom-' prising a resonant circuit tuned to the mean frequency, a plurality of electron emissiontubes having control electrodes, a capacitive circuit including one of said tubes connected in parallel with a portion of the resonant circuit, an inductive circuit includin another of said, tubesconnected in .paralle with a portion of the resonant circuit, and a modulating circuit coupled to the control i6 electrodes of said tubes in phase opposition.

21. In a transmitter for signalling by freto the intermediate frequency amplifier and tuned to a frequency lying outsidegof the de siredband of frequencies for chan 'n'g frequency modulation to am litude m ulation, means to rectify the amplitude modulated en- Z ergy, and means to translate the rectified energy.

' CLARENCE w. HANSELL.

quency wobble, in combination a source of a carrier energy, a source of low frequency energy, means to frequency wobble the carrier energy in response to the low frequency en ergy, means tovkey the low frequency ener in accordance with asignal'to be transmit'te -means to control the frequency of the low frequency energy, and means'other than the signalling means to control the wobble range.

22. In a system for communication by frequency modulation, in combination, a trans mitter having means other'than-the signalling'means to control the width of the fre g quency band, and a receiver having an analyzing circuit'means to control the sharpnessof as 24. In a frequency modulation receiver arranged to eliminate the effect of partial fading of frequency modulated signals, the combination of means to limit the signal energy to the low signal level, and means to thereo after change the frequenc'ymodulation of the limited si al energy tion for tection.

25. A receiving system for frequency modulated ener us a local oscil ator, means to combine the received and local energies to produce energy of intermediate frequency, anintermediate frequency amplifier coupled to the combining to amplitude modulameans and arranged to limit the signal transso ferred therethrough to the low'signal level,

an analyzing circuitcoupled to the intermediate frequency amplifier and tuned to a fre-' quency lying outside of the operating range of intermediate frequency, rectifying means, i6 and means to translate the rectified energy.

26. A receiving system for energy of constant amplitude, frequency modulated with relatively low frequency energy comprising a receiving circuit, a local oscillator, means to D0 combine the received and local energies to produce energy of intermediate frequency, a band pass filter and an intermediate frequency amplifier coupled to the combinin means for amplifying only the desired band I5 of frequencies, an analyzing circuit coupled comprising a receiving circuit,

DISCLAIMER 1,819,508.Olarence W. Hansell, Rocky Point, N. Y. COMMUNICATION BY FRE- QUENCY VARIATION. Patent dated August 18, 1931. Disclaimer filed December 11, 1935, by the assignee, Radio Corporation of America. Hereby enters this disclaimer with respect to claims 12 and 24 of the said patent, to wit:

12. The method of eliminating the effect of partial fading of frequency modulated signals which includes discriminating between amplitude and frequency variation by limiting the received signal to the low signal level, and thereafter changing frequency modulation to amplitude modulation for detection.

24. In a frequency modulation receiver arranged to eliminate the effect of partial fading of frequency modulated signals, the combination of means to limit the signal energy to the low signal level, and means to thereafter change the frequency modulation of the limited signal energy to amplitude modulation for detection.

[Ofiicial Gazette January 14, 1936.] 

